The invention relates to Coriolis flowmeters, and in particular, to methods and systems for measuring properties of a flow tube and of a material flowing through the flow tube.
Coriolis flowmeters measure mass flow and other information for fluids flowing through a flow tube in the flowmeter. Coriolis flowmeters are comprised of a Coriolis sensor and associated meter electronics. Exemplary Coriolis flowmeters are disclosed in U.S. Pat. No. 4,109,524 of Aug. 29, 1978, U.S. Pat. No. 4,491,025 of Jan. 1, 1985, and Re. 31,450 of Feb. 11, 1982, all to J. E. Smith et al. These flowmeters have one or more flow tubes of a straight or a curved configuration. Each flow tube configuration in a Coriolis flowmeter has a set of natural modes of vibration, which may be of a simple bending, twisting, torsional, or coupled type. Each flow tube is driven to oscillate at a resonance in one of these natural modes of vibration. Fluid flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter, is directed through the flow tube or flow tubes, and exits the flowmeter through the outlet side of the flowmeter. The natural vibration modes of the vibrating, fluid-filled system are defined in part by the combined mass of the flow tubes and the fluid flowing through the flow tubes.
When there is no flow through the flowmeter, all points along the flow tube oscillate, due to an applied driver force, with substantially identical phase or small initial fixed phase offset which can be corrected. As fluid begins to flow, Coriolis forces cause points along the flow tube to have a different phase. The phase on the inlet side of the flow tube commonly lags the driver, while the phase on the outlet side of the flow tube leads the driver. Pickoffs are affixed to the flow tube to measure the motion of the flow tube and to produce sinusoidal pickoff signals representative of the motion of the flow tube. The meter electronics processes pickoff signals to determine the phase difference between the pickoff signals. The phase difference between two pickoff signals is proportional to the mass flow rate of fluid through the flow tube.
An important component of Coriolis flowmeters, and of vibrating tube densitometers, is the drive or excitation system. The drive system operates to apply a periodic physical force to the flow tube which causes the flow tube to oscillate. The drive system includes a driver mechanism mounted to the flow tube of the flowmeter and a drive circuit for generating a drive signal to operate the driver mechanism. The driver mechanism typically contains one of many well known arrangements, such as a magnet mounted to one flow tube and a wire coil mounted to the another flow tube or brace bar in an opposing relationship to the magnet.
A drive circuit continuously applies a periodic drive voltage to the driver mechanism. The drive voltage is typically sinusoidally or square shaped. In a typical magnetic-coil drive mechanism, the periodic drive voltage causes the coil to produce a continuous alternating magnetic field. The alternating magnetic field of the coil and the constant magnetic field produced by the magnet force the flow tube to vibrate in a sinusoidal pattern. Those skilled in the art will recognize that any device capable of converting an electrical signal to mechanical force is suitable for application as a driver. (See, U.S. Pat. No. 4,777,833 issued to Carpenter and assigned on its face to Micro Motion, Inc.) Also, one need not use a sinusoidal signal but rather any periodic signal may be appropriate as the driver signal (See, U.S. Pat. No. 5,009,109 issued to Kalotay et. al. and assigned on its face to Micro Motion, Inc.).
For a dual tube flowmeter, a typical mode in which Coriolis flowmeters are typically driven to vibrate is a first out-of-phase bending mode. The first out-of-phase bending mode is the fundamental bending mode at which the two flow tubes of a dual tube Coriolis flowmeter vibrate in opposition to one another. However, this is not the only mode of vibration present in the vibrating structure of a Coriolis flowmeter driven in the first out-of-phase bending mode. Higher modes of vibration may also be excited in the flow tubes. For example, a first out-of-phase twist mode may be excited as a result of a fluid flowing through the vibrating flow tube and the Coriolis forces caused by the flowing fluid. Other higher modes of vibration that may be excited include in-phase and lateral modes of vibration. There may be hundreds of vibration modes actually excited in a Coriolis flowmeter that is driven to oscillate in the first out-of-phase bending mode. Even within a relatively narrow range of frequencies near the first out-of-phase bending mode, there are at least several additional modes of vibration that are excited by the vibration of the flow tube by the drive system. In addition to multiple modes being excited by the driver, additional undesired modes of vibration can also be excited due to vibrations external to the flowmeter. For example, a pump located elsewhere in a process line might generate a vibration along a pipeline that excites a mode of vibration in a Coriolis flowmeter.
As discussed above, the driver vibrates the flow tube at a resonant frequency. As the density of the fluid inside the flow tube changes, the resonant frequency changes. The change in resonant frequency squared is inversely proportional to the change in density, as described in the following equation:       Δ    ⁢          xe2x80x83        ⁢          f      2        =      K    Δρ  
where f represents the resonant frequency, K represents a proportionality constant, and xcfx81 represents density. The period (xcfx84) of the resonant frequency can also be used, as described in the following equation:
xcex94xcfx81=Kxcex94xcfx842
where xcfx84 represents the period of the resonant frequency. Users of Coriolis flowmeters may want to measure the absolute density rather than the relative change in fluid density. A calibration of the Coriolis flowmeter may be required to determine a proportionality constant K and a reference fluid density. Calibration of the Coriolis flowmeter is accomplished by measuring the frequency/period of resonance with two known fluids. The absolute fluid density can be calculated using the following equation:       ρ          m      ⁢              xe2x80x83            ⁢      e      ⁢              xe2x80x83            ⁢      a      ⁢              xe2x80x83            ⁢      s      ⁢              xe2x80x83            ⁢      u      ⁢              xe2x80x83            ⁢      r      ⁢              xe2x80x83            ⁢      e      ⁢              xe2x80x83            ⁢      d        =                    [                                            ρ              2                        -                          ρ              1                                                          τ              2              2                        -                          τ              1              2                                      ]            ⁢              (                                            τ                              m                ⁢                                  xe2x80x83                                ⁢                e                ⁢                                  xe2x80x83                                ⁢                a                ⁢                                  xe2x80x83                                ⁢                s                ⁢                                  xe2x80x83                                ⁢                u                ⁢                                  xe2x80x83                                ⁢                r                ⁢                                  xe2x80x83                                ⁢                e                ⁢                                  xe2x80x83                                ⁢                d                            2                        ⁢                          C              ⁡                              (                T                )                                              -                      τ            1            2                          )              +          ρ      1      
where xcfx841 and xcfx842 represent the tube period using two known fluids and xcfx811 and xcfx812 represent the densities of the two known fluids. C(T) is a temperature compensation for changes in the material of the Coriolis flowmeter due to temperature.
Unfortunately, the temperature of the fluid is often different than the ambient temperature around the flowmeter. The flow tube of the Coriolis flowmeter may grow or shrink due to thermal expansion. For a curved tube flowmeter, the thermal expansion may not be a problem because the flow tube is free to expand or shrink. For a straight tube flowmeter, the thermal expansion of the flow tube may be a problem because the flow tube is constrained from expansion along its axis by a case, a brace bar, or other means. The thermal expansion can result in a change in the resonant frequency due to temperature even though the fluid density may be unchanged. The meter electronics can compensate for the thermal expansion using a temperature correction, but the meter electronics have not been effectively adapted to handle thermal expansion by a more reliable means. This temperature correction is an indirect estimate of the tension/compression because it assumes a coefficient of thermal expansion.
Straight tube flowmeters are generally more sensitive to changes in boundary conditions than are curved tube flowmeters. Boundary conditions are the forces and moments that restrain the motion of a vibrating flow tube. Conversely, dual curved tube flowmeters are naturally counterbalanced, so the forces and moments exerted by the two flow tubes sum to zero. Some straight tube flowmeters utilize counterbalance systems to passively or actively oppose the boundary forces and moments exerted by a single flow tube. Passive counterbalance systems unfortunately only work well over a limited range of fluid densities. Active counterbalance systems add additional complexity to the flowmeter. Thus, problems caused by temperature changes and boundary condition changes are especially evident in straight tube flowmeters.
Properties of the flow tube and of the fluid flowing through the flow tube is useful information to obtain from a flowmeter. Properties of the flow tube and of the fluid flowing through the flow tube include the fluid density, the tension/compression in the flow tube, and flow tube""s material density, the pressure in the flow tube, and other properties. Unfortunately, accurate measurement of the properties of the flow tube and of the fluid flowing through the flow tube are currently difficult to obtain without compensating for conditions such as temperature changes and boundary condition changes.
The above and other problems are solved and an advance in the art is made by a system and method for determining properties of the flow tube and of the fluid flowing through the flow tube. The present invention determines the properties of the flow tube and of the fluid flowing through the flow tube without having to directly compensate for temperature changes and boundary condition changes in a straight tube flowmeter.
In accordance with this invention, meter electronics execute instructions that provide a process for determining properties of a flow tube and of a fluid flowing through the flow tube. The process begins when the meter electronics receives pickoff signals from a plurality of pickoffs. The meter electronics determines a measured mode shape of the flow tube based on the pickoff signals. The meter electronics then selects values for flow tube and fluid parameters. The flow tube and fluid parameters are any parameters that represent physical properties of a flow tube or of a fluid flowing through the flow tube. The meter electronics then determines an estimated mode shape of the flow tube based on the values for the flow tube and fluid parameters. The meter electronics compares the estimated mode shape to the measured mode shape to determine an error for the values for the flow tube and fluid parameters. The meter electronics determines if the error for the values for the flow tube and fluid parameters is within an error range. If the error for the values is within the error range, then the meter electronics determines the properties of the flow tube and of the fluid flowing through the flow tube based on at least one of the values for the flow tube and fluid parameters.
In some examples, if the error for the values is not within the error range, then the meter electronics selects new values for the flow tube and fluid parameters. The meter electronics then repeats the above process using the new values.
In some examples, one of the properties of the flow tube and of the fluid being determined is the density of the fluid. In order to determine the density, the meter electronics may have to determine one or more density calibration factors. The determination of the density calibration factors may include flowing a first fluid of a known density through the flow tube. The meter electronics receives pickoff signals indicating motion of the flow tube as the first fluid flows through the flow tube. The determination further includes flowing a second fluid of a known density through the flow tube. The meter electronics receives pickoff signals indicating motion of the flow tube as the second fluid flows through the flow tube. The meter electronics determines the density calibration factors from the pickoff signals received responsive to the first and the second fluids flowing through the flow tube.
One aspect of the invention includes a method for determining properties of a flow tube and of a fluid flowing through said flow tube in response to receiving pickoff signals from a plurality of pickoffs associated with said flow tube, said pickoff signals indicating vibrations of said flow tube vibrated by a driver associated with said flow tube, said method comprising the steps of:
a) receiving said pickoff signals from said plurality of pickoffs;
b) determining a measured mode shape of said flow tube based on said pickoff signals;
c) selecting values for flow tube and fluid parameters;
d) determining an estimated mode shape of said flow tube based on said flow tube and fluid parameters;
e) comparing said estimated mode shape to said measured mode shape to determine an error for said values for said flow tube and fluid parameters; and
f) if said error for said values for said flow tube and fluid parameters is within an error range, then:
determining said properties of said flow tubes and of said fluid flowing through said flow tube based on said values for said flow tube and fluid parameters.
Another aspect of the invention includes a method further comprising:
(g) if said error for said values for said flow tube and fluid parameters is not within said error range, then:
selecting new values for said flow tube and fluid parameters; and
repeating steps (d)-(g).
Another aspect of the invention includes a method wherein the step of determining said properties of said flow tube and of said fluid flowing through said flow tube comprises:
determining a density of said fluid flowing through said flow tube based on said values for said flow tube and fluid parameters.
Another aspect of the invention includes a method further comprising the steps of:
flowing a first fluid of a known density through said flow tube and receiving said pickoff signals indicating motion of said flow tube as said first fluid flows through said flow tube to generate first factors;
flowing a second fluid of a known density through said flow tube and receiving said pickoff signals indicating motion of said flow tube as said second fluid flows through said flow tube to generate second factors; and
determining density calibration factors based on said first and second factors;
wherein said step of determining said density of said fluid flowing through said flow tube further comprises determining said density of said fluid flowing through said flow tube based on said values for said flow tube and fluid parameters and said density calibration factors.
Another aspect of the invention includes a method wherein a first one of said density calibration factors comprises a ratio of area per unit length of said fluid to a flexural rigidity of said flow tube.
Another aspect of the invention includes a method wherein a second one of said density calibration factors comprises a ratio of mass per unit length of said flow tube to said flexural rigidity of said flow tube.
Another aspect of the invention includes a method wherein:
a first one of said values for said flow tube and fluid parameters comprises a ratio of mass per unit length of said fluid and said flow tube to said flexural rigidity of said flow tube; and
said step of determining said density of said fluid flowing through said flow tube comprises:
subtracting said second one of said density calibration factors from said first one of said values for said flow tube and fluid parameters to yield a first result; and
multiplying said first result by an inverse of said first one of said density calibration factors to determine said density of said fluid flowing through said flow tube.
Another aspect of the invention includes a method wherein said plurality of pickoffs comprises at least four boundary condition pickoffs affixed to said flow tube and configured to generate said pickoff signals.
Another aspect of the invention includes a method wherein said plurality of pickoffs further comprises at least one reference pickoff affixed to said flow tube and configured to generate a reference signal.
Another aspect of the invention includes a method wherein said step of determining said new values for said flow tube and fluid parameters comprises comparing said flow tube and fluid parameters from at least two modes of vibration of said flow tube to determine said new values.
Another aspect of the invention includes meter electronics configured to determine properties of a flow tube and of a fluid flowing through said flow tube in response to receiving pickoff signals from a plurality of pickoffs associated with said flow tube, said signal indicating vibrations of said flow tube being vibrated by a driver associated with said flow tube, said meter electronics comprising: a processing unit configured to read instructions from a storage media; and said instructions configured to direct said processing unit to:
a) receive said pickoff signals from said plurality of pickoffs;
b) determine a measured mode shape of said flow tube based on said pickoff signals;
c) select values for flow tube and fluid parameters;
d) determine an estimated mode shape of said flow tube based on said flow tube and fluid parameters;
e) compare said estimated mode shape to said measured mode shape to determine an error for said values for said flow tube and fluid parameters; and
f) if said error for said values for said flow tube and fluid parameters is within an error range, then:
determine said properties of said flow tube and of said fluid flowing through said flow tube based on said values for said flow tube and fluid parameters.
Another aspect of the invention includes meter electronics wherein said instructions are further configured to direct said processing unit to:
(g) select new values for said flow tube and fluid parameters; and
repeat steps (d)-(g) if said error for said values for said flow tube and fluid parameters is not within said error range.
Another aspect of the invention includes meter electronics wherein said instructions are further configured to direct said processing unit to:
determine a density of said fluid flowing through said flow tube based on said values for said flow tube and fluid parameters.
Another aspect of the invention includes meter electronics wherein said instructions are further configured to direct said processing unit to:
generate first factors in response to receiving said pickoff signals indicating motion of said flow tube as a first fluid of a known density flows through said flow tube;
generate second factors in response to receiving said pickoff signals indicating motion of said flow tube as a second fluid of a known density flows through said flow tube;
determine density calibration factors based on said first and second factors; and
determine said density of said fluid flowing through said flow tube based further on said density calibration factors.
Another aspect of the invention includes meter electronics wherein a first one of said density calibration factors comprises a ratio of area per unit length of said fluid to a flexural rigidity of said flow tube.
Another aspect of the invention includes meter electronics wherein a second one of said density calibration factors comprises a ratio of mass per unit length of said flow tube to said flexural rigidity of said flow tube.
Another aspect of the invention includes meter electronics wherein:
a first one of said values for said flow tube and fluid parameters comprises a ratio of mass per unit length of said fluid and said flow tube to said flexural rigidity of said flow tube; and
wherein said instructions that are configured to direct said processing unit to determine said density of said fluid are further configured to direct said processing unit to:
subtract said second one of said density calibration factors from said first one of said values for said flow tube and fluid parameters to yield a first result; and
multiply said first result by an inverse of said first one of said density calibration factors to determine said density of said fluid flowing through said flow tube.
Another aspect of the invention includes meter electronics wherein said plurality of pickoffs comprises at least four boundary condition pickoffs affixed to said flow tube and configured to generate said pickoff signals.
Another aspect of the invention includes meter electronics wherein said plurality of pickoffs further comprises at least one reference pickoff affixed to said flow tube and configured to generate a reference signal.
Another aspect of the invention includes meter electronics wherein said instructions that are configured to direct said processing unit to determine said new values for said flow tube and fluid parameters are further configured to direct said processing unit to compare said flow tube and fluid parameters from at least two modes of vibration of said flow tube to determine said new values.